Solid state masers, where the maser gain medium is in solid state form are known. Typically, however, such solid state masers require operation in the ultra low temperature regime. Ruby has been used as a solid state gain medium for a solid state medium operating at about 60° K [C. R. Ditchfield and P. A. Forrester, ‘Maser action in the region of 60° K’ Phys. Rev. Letts., 1, p 448 (1958)].
More recently, organic masers have been demonstrated at room temperature by Oxborrow et al. [M. Oxborrow, J. D. Breeze and N. M. Alford, ‘Room-temperature solid-state maser’, Nature, 488, pp 353-356 (2012)]. The power output for such an organic maser was only −10 dBm at a masing frequency of 1.45 GHz.